Magnetic field sensor

ABSTRACT

A magnetic field sensor for measuring at least two components of a magnetic field comprises a ferromagnetic core mounted on a semiconductor chip, an exciter coil to which a current can be applied and two read-out sensors. The ferromagnetic core is ring-shaped. The exciter coil is preferably formed from conductor tracks of the semiconductor chip and from bonding wires.

CROSS REFERENCE TO RELATED APPLICATION

The present application is related to and claims priority of thefollowing co-pending applications, namely, PCT application numberPCT/EP02/05559 of Sentron AG entitled Magnetic Field Sensor, filed onMay 21, 2002; and EP Application No. 01810518.9, filed May 25, 2001. Theabove-identified applications are incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The invention concerns a magnetic field sensor.

BACKGROUND OF THE INVENTION

Such magnetic field sensors are suited for the measurement of magneticfields the strength of which are only a few nT to mT for example as acompass for measuring the direction of the earth's magnetic field.

A magnetic field sensor of the type named in the preamble of claim 1 isknown from EP 1 052 519 A1 as well as from the article “CMOS planar 2Dmicro-fluxgate sensor” by the authors L. Chiesi, P. Kejik, B. Janossyand R. S. Popovic, which was published in the magazine Sensors andActuators 82 (2000) 174–180. Such a magnetic field sensor contains aferromagnetic core and an excitation coil through which alternatingcurrent flows in order to alternately magnetically saturate anddemagnetise the core. An important disadvantage of this sensor exists inthat an adequate sensitivity has to be taken at the expense of arelatively large ferromagnetic core. This stands in the way of a furtherminiaturisation and makes the sensor relatively expensive. A furtherproblem exists in that the ferromagnetic core can be unintentionallymagnetised by an outer magnetic field that is much larger than theactual field to be measured. The current flowing in the coil is then nolonger able to freely align the individual magnetic domains, which leadsto a measuring error.

A device for measuring one single component of a magnetic field is knownfrom EP 359 922 A1 with which a ring-shaped ferromagnetic core serves tochop the magnetic field the direction of which is fixed. Theferromagnetic core however does not serve as a flux concentrator.

SUMMARY OF THE INVENTION

The object of the invention is to develop a concept for magnetic fieldsensors, which enables a further miniaturisation.

The invention assumes the principle of a fluxgate sensor as is known,for example, from the above-mentioned article. Fluxgate sensors have anexciter coil, a ferromagnetic core and a read-out coil. They aresuitable for the measurement of weak magnetic fields as the magneticfield to be measured is chopped with the aid of the exciter coil and theferromagnetic core, ie, is periodically switched on and off at thelocation of the read-out coil. The output signal of the read-out coilcan then be evaluated synchronously to the chopping with the lock-intechnique. The invention suggests that, for optimising thecharacteristics of a magnetic field sensor foreseen for the measurementof weak magnetic fields, all processes used in the semiconductortechnology are included, ie, all processes from the start of theproduction of the semiconductor chip on a wafer over the post-processingwhere the ferromagnetic core is mounted, up to the backend where thesemiconductor chip is mounted and encapsulated to the finished magneticfield sensor. This approach enables the use of a ring-shapedferromagnetic core the advantage of which exists in that it can bemagnetically saturated with a minimum of electric current and electricalpower. The windings of the exciter coil then preferably consist ofconductor tracks and bonding wires. In addition, the invention suggeststhat Hall elements be used instead of read-out coils because their sizecan be reduced as desired without loss in sensitivity. Such a magneticfield sensor can be greatly miniaturised.

In the following, embodiments of the invention are explained in moredetail based on the drawing.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

It is shown in:

FIG. 1 a magnetic field sensor with an exciter coil and a ferromagneticcore for chopping the measuring field,

FIG. 2A, B field lines of a magnetic field,

FIG. 3 a magnetic field sensor with an exciter coil with severalwindings,

FIG. 4 a magnetic field sensor mounted as a flipchip,

FIG. 5 a further magnetic field sensor,

FIG. 6 a magnetic field sensor with an exciter coil formed as a flatcoil,

FIG. 7 details of a further exciter coil, and

FIG. 8 a fluxgate sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a plan view of a magnetic field sensor formed as asemiconductor chip 1 for the measurement of two components of a magneticfield. A cartesian x,y,z system of co-ordinates serves as referencesystem whereby the z direction runs vertically to the drawing plane. Themagnetic field sensor comprises an exciter coil 2 with at least onewinding to which current is applied, a ring-shaped ferromagnetic core 4,two read-out sensors 5, 6 and an electronic circuit 7. The read-outsensor 5 serves to detect the x component of the magnetic field, theread-out sensor 6 serves to detect the y component of the magneticfield. Preferably, each of the read-out sensors 5, 6 consists of twolocally separated but electrically connected sensors. The magnetic fieldsensor is produced using a technology with which the electronic circuit7, parts of the exciter coil 2 and the read-out sensors 5, 6 are firstmanufactured using a standard CMOS technology and then the ferromagneticcore 4 is applied in a so-called post-process. In doing so, a tape madeof amorphous ferromagnetic material is glued onto the wafer with thesemiconductor circuits and structured by means of photolithography andchemical etching. After sawing the wafer into the individualsemiconductor chips, the at least one winding of the exciter coil 2 iscompleted on mounting the semiconductor chip onto a substrate 8 eitherby wire bonding or by means of the flipchip technology. The electroniccircuit 7 serves to produce the current flowing through the exciter coil2 and the evaluation of the signals delivered by the read-out sensors 5,6.

With the embodiment shown in FIG. 1, the exciter coil 2 has one singlewinding which consists partially of a first conductor track 9 and asecond conductor track 10 and partially of two bonding wires 11 and 12.The first conductor track 9 leads from a current source 13 arrangedoutside the ring-shaped ferromagnetic core 4 under the ferromagneticcore 4 to a first bond pad 14 arranged inside the ring-shapedferromagnetic core 4. The first bonding wire 11 leads from the firstbond pad 14 to a bond pad 15 on the substrate 8. The second bonding wire12 leads from this bond pad 15 to a second bond pad 16 arranged on thesemiconductor chip 1 outside the ring-shaped ferromagnetic core 4. Thesecond conductor track 10 leads from the second bond pad 16 to thecurrent source 13.

As material for the ferromagnetic core 4 serves, for example, the tapeavailable under the designation VAC 6025Z made out of amorphous metal.This material has a coercive field strength of H_(c)=3 mA/cm. In orderto magnetically saturate the ferromagnetic core 4, the current I flowingthrough the exciter coil 2 should produce a magnetic field H_(s) that isaround 20 times greater than the coercive field strength H_(c). When theouter diameter D of the ring-shaped ferromagnetic core 4 amounts toD=300 μm and the number n of the windings of the exciter coil 2 amountsto n=1, then, according to the equationI=20*H _(c) *D*π/n  (1)one gets I≅6 mA for the current. Furthermore, the duty cycle of thecurrent I can be reduced to around 10% through which the average currentrequirement reduces to 0.6 mA. Because the ferromagnetic core 4 has noair gap, it can already be magnetically saturated by a small magneticfield and therefore by a low current I.

As read-out sensors 5 and 6 the magnetic field sensor has four so-calledhorizontal Hall elements 17, 18, 19 and 20 electrically coupled in pairsthat are sensitive to a magnetic field that runs vertically to thesurface of the semiconductor chip 1, ie, in z direction. The Hallelements 17 and 19 are arranged on the x axis of the cartesian system ofco-ordinates and form the first read-out sensor 5, the Hall elements 18and 20 are arranged on the y axis of the cartesian system ofco-ordinates and form the second read-out sensor 6. The Hall elements 17to 20 are each arranged underneath the ferromagnetic core 4 and, infact, close to the outer edge of the ferromagnetic core 4.

In operation, the magnetic field sensor works as follows: A preferablysquare-wave shaped alternating current from the current source 13 isapplied to the exciter coil 2. In doing so, the alternating currentsaturates and demagnetises the ferromagnetic core 4 with the frequencyof the alternating current. In the phase where the ferromagnetic core 4is magnetically saturated, it has no effect on the external magneticfield to be measured. The field lines of the magnetic field run parallelto the surface of the Hall elements 17 to 20: The Hall elements deliverno output signal. In the phase where the ferromagnetic core 4 isdemagnetised, it has the effect of a magnetic flux concentrator on themagnetic field to be measured. Because the relative permeability μ_(r)of the ferromagnetic core 4 is very large in comparison to the relativepermeability of its surroundings, the field lines of the magnetic fieldhit the surface of the ferromagnetic core 4 almost vertically and leaveit at an almost vertical angle. The field concentration is greatest inthe area of the edge of the ferromagnetic core 4 where the Hall elements17 to 20 are located. The output signals from at least two of the fourHall elements 17 to 20 then differ from zero.

FIG. 2A shows the field lines 21 of the magnetic field when theferromagnetic core 4 is saturated and when the magnetic field runs alongthe x direction and therefore parallel to the surface of thesemiconductor chip 1. Only the ferromagnetic core 4 and two read-outsensors 5 are presented. The sectional plane of the figure runsvertically to the surface of the semiconductor chip 1 and parallel tothe outer magnetic field. FIG. 2B shows the field lines 22 of the samemagnetic field when the ferromagnetic core 4 is not saturated by thecurrent flowing through the exciter coil. The field lines at thelocation of the two read-out sensors 5 point in a different z directionbecause the magnetic field enters the ferromagnetic core 4 at thelocation of one of the read-out sensors 5 (in FIG. 2B for example theleft read-out sensor 5) and leave it again at the location of the otherread-out sensor 5 (in FIG. 2B the right read-out sensor 5). The tworead-out sensors 5 are correspondingly switched electrically.

The exciter coil 2 therefore serves to use the ferromagnetic core forchopping the magnetic field to be measured. The output signals of theHall elements 17 to 20 can then be evaluated synchronously with thecurrent flowing through the exciter coil 2 by means of the known lock-intechnique.

In the following, further measures are now described the application ofwhich leads to a higher sensitivity of the magnetic field sensor and/orto a lower current or energy consumption.

When the number of windings of the exciter coil 2 is increased to n,then, with the same energy consumption, either the current flowingthrough the exciter coil 2 can be reduced by the factor n or thediameter D of the ring-shaped ferromagnetic core 4 can be increased bythe factor n. An increase in the diameter D has the effect ofstrengthening the flux concentration but also increases the spacerequirement and therefore the dimensions of the semiconductor chip 1.With regard to the aim of the greatest possible miniaturisation of themagnetic field sensor, optimum relationships are then achieved when thediameter D of the ferromagnetic core 4 is adapted to the dimensions ofthe semiconductor chip 1, which result from the space requirement forthe electronic circuit 7, and the number n of windings 3 is adapted tothe size of the ferromagnetic core 4.

FIG. 3 shows a plan view of a magnetic field sensor the exciter coil 2of which has four windings connected in series. Each of the windingsconsists of a conductor track 23 and a bonding wire 24, whereby eachbonding wire 24 leads from a bond pad 25 within the ferromagnetic core 4to a bond pad 26 outside the ferromagnetic core 4. With this example,all bond pads are located on the semiconductor chip 1.

FIG. 4 shows a cross-section of a magnetic field sensor mounted as aflipchip with which the windings of the exciter coil are realised bymeans of conductor tracks 23 on the semiconductor chip 1, so-calledbumps 27 and printed conductors 28 on the substrate 8.

With the magnetic field sensors described, the z components of themagnetic field can also be measured. Here, as opposed to a magneticfield running parallel to the surface of the semiconductor chip 1, thefield lines of the magnetic field point in the same direction at allread-out sensors 5, 6 or Hall elements 17 to 20. The Hall elements 17 to20 then have to be connected corresponding to this condition. Formeasurement of the components of the magnetic field running horizontallyto the surface of the semiconductor chip 1, ie, the x and y components,the difference in the output voltage of the two Hall elements 17 and 19or the two Hall elements 18 and 20 has to be determined while formeasurement of the z components running vertically to the surface of thesemiconductor chip 1 the sum of the output voltages of the Hall elements17 to 20 has to be determined.

As long as the thickness of the ferromagnetic core 4, ie, its dimensionin x direction, is comparatively small in comparison with its width, theferromagnetic core 4 does not work as a flux concentrator for the zcomponent of the magnetic field. When however the thickness correspondsto its width, then the ferromagnetic core 4 also works as a fluxconcentrator for the z component of the magnetic field. The magneticfield can then also be chopped for measurement of the z component bymeans of the current flowing through the exciter coil which considerablyincreases the sensitivity of the magnetic field sensor for the zcomponent. The thickness of the ferromagnetic core 4 then preferablyamounts to at least 0.5 times its width.

The current flowing through the exciter coil 2 can also be used tosupply the Hall elements 17 to 20 as the ohmic resistance of the excitercoil 2 is low.

A further measure for increasing the efficiency of the magnetic fieldsensor consists in designing the ferromagnetic core 4 in such a way thatthe ferromagnetic core 4 can be magnetically saturated locally withoutthe entire ferromagnetic core 4 being magnetically saturated. Themagnetic field is then still sufficiently chopped in the area of theread-out sensors 5, 6, i.e., the Hall elements 17 to 20. FIG. 5 shows aplan view of a magnetic field sensor modified in this way. Thering-shaped ferromagnetic core 4 with the diameter D contains four holes29 so that four small rings 30 are created each of which is arrangedbetween two of the Hall elements 17 to 20. As an example, the large ringhas a diameter of D=1.5 mm and the small rings 30 a diameter of d=150μm. A bondpad 31 is located within each of the small rings 30. Such asmall ring 30 presents a closed magnetic circuit. Again, the excitercoil consists of conductor tracks 23 and bonding wires 24 that are laidout and wired so that the current flowing through the exciter coilsaturates the small rings 30. The current flowing through the excitercoil produces a magnetic field in each of the small rings 30 whichmagnetically saturates the small ring 30. Independently of the diameterD of the large ring, when using the metal tape VAC 6025Z a current I≅3mA calculated in accordance with equation (1) is sufficient tomagnetically saturate the small rings 30. When the small rings 30 aresaturated, then the large ring loses its function as flux concentrator:The external magnetic field is chopped.

FIG. 6 shows a plan view of a section of a magnetic field sensor withwhich the exciter coil is designed in the form of a flat coil 32. Theflat coil 32 occupies a part of the space underneath the ring-shapedferromagnetic core 4. Because the magnetic field lines produced by theflat coil 32 can not be closed within the ferromagnetic core 4,interference fields are created. The dimensions of the flat coil 32should therefore be small in comparison with the diameter D of theferromagnetic core 4 and the flat coil 32 should be as far away aspossible from the read-out sensors 5, 6 (or Hall elements 17–20). Theflat coil 32 consists of two coil sections wound in opposite directionsof which one coil section is essentially arranged within theferromagnetic core 4, the other coil section is arranged outside theferromagnetic core 4. The coil sections, as presented in FIG. 6, arewound so that the current flowing through the winding sections runningclose to the ferromagnetic core 4 flows in the same direction. Insteadof one single flat coil, several flat coils can also be arranged alongthe ferromagnetic core 4.

With today's CMOS processes, several metal layers are customary. Thesemetal layers can be used to realise the flat coils as long as they arenot required for the electronic circuit 7.

The suggestions according to FIGS. 5 and 6 can be combined. FIG. 7 showsa section of the magnetic field sensor according to FIG. 5 comprisingonly one of the small rings 30. A flat coil 33 is arranged underneaththe small ring 30 the windings of which circle spirally around the bondpad 31. With this solution, the exciter coil comprises the flat coil 33and the windings formed from the conductor track 23 and the bonding wire24.

When a great as possible miniaturisation of the magnetic field sensor isaimed for, then the form of the read-out sensors 5, 6 as Hall elementsis the right choice. When on the other hand a highest possiblesensitivity is aimed for so that extremely small magnetic fields canalso be measured, then it is necessary on the one hand to make thediameter D of the ring-shaped ferromagnetic core 4 relatively large and,on the other hand, it is useful to choose flat coils as read-out sensors5, 6. The sensitivity of the flat coils increases square to theavailable space and therefore square to the diameter D while thesensitivity of the Hall elements is independent of their size andtherefore only increases linear to the diameter D. Therefore, with allthe embodiments shown, the Hall elements can be replaced by flat coils.As an example, FIG. 8 shows a plan view of a magnetic field sensor withflat coils 34 to 37 which serve as read-out sensors 5. For reasons ofclarity, the exciter coil is not drawn. The flat coils 34 to 37 arewound so that their windings encircle the magnetic field which, as shownin FIG. 2B, runs vertically to their surface. The flat coils 34 to 37are preferably square-shaped or their shape is adapted to the curvatureof the ring so that their windings encircle a largest possible sectionof the ferromagnetic core 4 and therefore a largest possible part of themagnetic flux. The read-out sensor 5 comprises the two flat coils 34 and36, which are wound in opposite directions so that their signals areadded. The read-out sensor 6 comprises the two flat coils 35 and 37. Theflat coils 34 to 37 are connected to an evaluating circuit 38. Such amagnetic field sensor is also designated as a fluxgate sensor.

With the embodiments up to now, horizontal Hall elements have been used.However vertical Hall elements can also be used. These are however notto be arranged underneath the ferromagnetic ring but outside theferromagnetic ring in the area of its outer edge. With the example inFIG. 3 the ideal position of the vertical Hall elements results frommirroring the position of the horizontal Hall elements 17–20 on theouter edge of the ferromagnetic core 4. Vertical Hall elements arenamely sensitive to a magnetic field that runs parallel to theirsurface. The course of the field lines to be measured can be seen inFIG. 2B. The ideal position of the vertical Hall elements is thereforelocated next to the outer edge of the ferromagnetic core 4.

1. A magnetic field sensor for measuring at least two components of amagnetic field, comprising a ferromagnetic core for concentrating themagnetic field, mounted on a surface of a semiconductor chip, having agenerally flat shape extending substantially in a first plane and havingan inner and an outer edge, the inner edge forming a boundary of anopening, two read-out sensors for measuring a first and a secondcomponent of the magnetic field in said plane, the read-out sensorsintegrated in the semiconductor chip and located close to the outer edgeof the ferromagnetic core, an exciter coil having at least one winding,a part of the winding formed by a conductor track of the semiconductorchip and another part of the winding running outside the surface of thesemiconductor chip, and a current source for applying an AC current tothe exciter coil, the exciter coil and the ferromagnetic core servingfor chopping the first and second component of the magnetic field. 2.The magnetic field sensor according to claim 1, wherein theferromagnetic core has at least one hole located between the inner andthe outer edge and wherein at least one winding of the exciter coilleads through this hole so that the current flowing through the excitercoil can magnetically saturate the ferromagnetic core in the area of theat least one hole without saturating the remaining areas of theferromagnetic core.
 3. The magnetic field sensor according to claim 1,wherein each of the read-out sensors comprises two horizontal Hallelements that are arranged diametrically to one another on oppositesides of the ferromagnetic core and underneath the ferromagnetic core.4. The magnetic field sensor according to claim 2, wherein each of theread-out sensors comprises two horizontal Hall elements that arearranged diametrically to one another on opposite sides of theferromagnetic core and underneath the ferromagnetic core.
 5. Themagnetic field sensor according to claim 1, wherein each of the read-outsensors comprises a vertical Hall element that is arranged outside theouter edge of the ferromagnetic core.
 6. The magnetic field sensoraccording to claim 2, wherein each of the read-out sensors comprises avertical Hall element that is arranged outside the outer edge of theferromagnetic core.
 7. The magnetic field sensor according to claim 1,for measuring three components of the magnetic field, wherein athickness of the ferromagnetic core amounts to at least 0.5 times itswidth.
 8. The magnetic field sensor according to claim 2, for measuringthree components of the magnetic field, wherein a thickness of theferromagnetic core amounts to at least 0.5 times its width.
 9. Themagnetic field sensor according to claim 3, for measuring threecomponents of the magnetic field, wherein a thickness of theferromagnetic core amounts to at least 0.5 times its width.
 10. Themagnetic field sensor according to claim 4, for measuring threecomponents of the magnetic field, wherein a thickness of theferromagnetic core amounts to at least 0.5 times its width.
 11. Themagnetic field sensor according to claim 5, for measuring threecomponents of the magnetic field, wherein a thickness of theferromagnetic core amounts to at least 0.5 times its width.
 12. Themagnetic field sensor according to claim 6, for measuring threecomponents of the magnetic field, wherein a thickness of theferromagnetic core amounts to at least 0.5 times its width.